1. Field of the Invention
The present invention relates to a technology of judging a sensor position utilizing a position-direction measuring apparatus which executes measurement by setting a hemisphere (semi-sphere) with a transmitter positioned at the center thereof and utilizing an assumption that a sensor is present within such hemisphere.
2. Related Background Art
In a virtual reality system or a composite reality system, it is commonly executed to acquire position and direction of a hand or a view position, obtained by a position direction measuring apparatus, and utilizing such information for a synthesis of computer graphics (CG).
As a position direction measuring apparatus of magnetic type, there are known FFASTRAK of Polhemus Corp., U.S. and Flock of Birds of Ascension Corp., U.S.
Such sensor of magnetic type is composed of a transmitter generating a magnetic field, a sensor to be attached to an object of which position and direction are to be measured, and a controlling main body, but, in the position measurement, is in principle incapable of recognizing where the sensor is actually present among two points which is point-symmetrical with respect to the position of the transmitter.
For this reason, the measurement is generally executed by setting a hemisphere around the transmitter and assuming that the sensor is present within such hemisphere. For example a plane as shown in FIG. 10 is taken as a boundary, and the measurement is executed only in a side indicated by an arrow. In such example, the transmitter is assumed to be present at the original point. Therefore the measurable area is inevitably halved.
A manual for the Flock of Birds of Ascension Corp. describes a method of determining the hemisphere in which the sensor is present by tracking the sensor from the past measured values, thereby achieving measurement in the entire sphere.
FIG. 9 shows a flow of such process, and such process will be explained with an example of measurement shown in FIGS. 7 and 8.
In this measurement, it is assumed that, in an initial state, the sensor is positioned in an upper hemisphere where the Y-value is positive. Even in case the sensor moves to a position where the Y-value is negative, measured values are outputted in point-symmetrical values with respect to the transmitter, namely in inverted values in X, Y and Z.
At first the measurement is initiated by placing the sensor in a position where the Y-value is positive. In this state, the output value from the measuring device coincides with the true value (in hemisphere) (step S2201).
When the measurement is started, a position is detected and position data are acquired from the measuring device (step S2202).
Then, there is discriminated whether signals other than the Y-signal which is assumed to remain positive, namely X-signal and Z-signal, have simultaneously changed signs thereof, and, a shift to another hemisphere is judged to have taken place in case of simultaneous changes of the signs (step S2203). In case of a shift of the hemisphere, there is memorized information indicating a new hemisphere. It is thus possible to memorize the hemisphere judged in the step S2203 (step S2204).
Then there is discriminated whether the hemisphere determined in the step S2203 is the hemisphere assumed by the measuring device (initial hemisphere) (step S2205), and, if it is different from the initial hemisphere, the measured values are converted to point-symmetrical values by respectively inverting the X-, Y- and Z-values (step S2206), and stored as a current position (step S2207). On the other hand, if it is same as the initial hemisphere, the X-, Y- and Z-values are stored as a current position.
The process of the steps S2202 to S2207 will be explained in an example shown in FIG. 7.
Now, let us assume a case where the sensor is moved from A to B. Since the position sensor assumes that the Y-value is positive, the measuring device outputs values of a position B′. In a position shift from A to B′, the X-value and the Z-value are simultaneously inverted, so that there can be judged a shift to another hemisphere, namely from A to B, and the position B can be judged as the correct measured value.
However, such discrimination on the shift between the hemispheres may involve an error. More specifically, the sensor may be so moved as to simultaneously change the signs on two axes of which sign is not assumed as positive nor negative.
Referring to FIG. 7, in case the sensor moves from A to B′ instead of B, the X-value and the Z-value are inverted simultaneously. Therefore, in case the hemisphere is judged by the conventional method shown in FIG. 9, the sensor is judged to be in a position B which is different from the actual position.
Also since the respective signs of X, Y and Z values are used for discriminating the initial hemisphere, the measurement start position of the sensor for determining the initial hemisphere cannot be set in a hemisphere which is inclined to the coordinate system as shown in FIG. 10 but there has to be selected a hemisphere of which each of X-, Y- and Z-values is fixed as positive or negative (namely a hemisphere at above, at below, at right, at left, in front or at rear), so that the installation or operation of the transmitter is limited.